... If you mount the origin device with missing snapshot, you destroy the
snapshot (even if you don't touch it). The snapshot can no longer be
repaired.

So it is safer to not activate device in this case then destroy data.

Why? What value is the old snapshot at this point? You just had a system
reboot in the middle of a snapshotted backup so all you need to do is get the
system up, redo another snapshot and retake your backup. I'm not interested
in the old snapshot.

If you use snapshots for something other than backups (for example
version-control using snapshots --- to enable admin to revert changes if
he messes something), then the snapshot is valuable and should survive
reboot.

Anyway, placing a ramdisk to volume group is bad idea and it must not be
done on any production system --- note that any lvcreate, lvconvert, etc
command can allocate anything on that ramdisk --- without the
administrator knowing it.
--- so I don't see any reason why we should do extra hacks to lvm for
people who placed ramdisk into vg.

To use temporary storage for snapshot, a special command for lvm would be
more appropriate --- a command that would setup snapshot and write nothing
about it to metadata, so that the snapshot would be forgotten on next
reboot --- then, you can setup the snapshot on any device outside the
volume group. You can already do this with dmsetup.